The present invention relates to a copier which is equipped with a toner supply device.
A copier having a toner cartridge which is filled with a two-component developer, i.e., a mixture of toner and carrier is extensively used today. In a copier with a toner supply device which includes such a toner cartridge, a control is so performed as to allow a predetermined extra number of copies to be produced even after a toner-end condition has been sensed and then to disable the copier. When a main switch or a front cover of the copier is manipulated after the copier has been disabled, the copier is restored to a condition in which it is ready to produce a predetermined number of copies which may be urgently needed. More specifically, the copier becomes ready to produce copies when the main switch is turned on or off or the front door is opened or closed without the need for supplying toner, i.e., replacing the old or empty toner cartridge with a new or full toner cartridge. This, however, brings about a problem that the density of toner and therefore that of image is sequentially lowered and, in addition, carrier is apt to adhere to a photoconductive element to damage it and/or a cleaning unit.
There has also been known a copier of the type forming an exclusive pattern on a photoconductive element for controlling toner density. In this type of copier, when a decrease in the density of the exclusive pattern beyond a reference level is sensed, it is decided that toner has ended. After a predetermined extra number of copies have been produced in such a toner end condition, the copier is disabled. This type of copier is also brought into a condition for producing a predetermined number of copies after it has been disabled when its main switch or front cover is operated. Such a prior art copier has various drawbacks in addition to those of the previously stated one which does not use an exclusive pattern. Since toner is supplied after toner density has been lowered, the density of reproduced images remains low until the toner density is restored to normal. Since toner density is sensed indirectly, even the fatigue of a photoconductive element, developer and the like, the failure of a charging unit or similar occurrence is sometimes sensed as being representative of the end of toner, resulting in accurate detection.